> This example is lacking indexes on the child tables, which is
> why the plan shown is about as good as you're going to get.
> The contents of foo1 and foo2 have to be read in entirety in any
> case, and sorting them separately is not a win compared to doing
> a single sort.
It is true, but not in case of FDW connected to remote host.
In this case sending large volumes of data through network will be very
inefficient.
There will be no problem if FDW can provide index scan - in this case
MergeAppend will fetch only required number of records:
postgres=# explain analyze select * from t order by u limit 1;
QUERYPLAN
-----------------------------------------------------------------------------------------------------------------------
Limit (cost=300.17..300.23 rows=1 width=8) (actual time=4.588..4.588
rows=1 loops=1) -> Merge Append (cost=300.17..762.76 rows=7681 width=8) (actual
time=4.586..4.586 rows=1 loops=1) Sort Key: t.u -> Index Scan using t_pkey on t (cost=0.12..8.14
rows=1
width=8) (actual time=0.003..0.003 rows=0 loops=1) -> Foreign Scan on t_fdw1 (cost=100.00..193.92 rows=2560
width=8) (actual time=1.532..1.532 rows=1 loops=1) -> Foreign Scan on t_fdw2 (cost=100.00..193.92 rows=2560
width=8) (actual time=1.510..1.510 rows=1 loops=1) -> Foreign Scan on t_fdw3 (cost=100.00..193.92 rows=2560
width=8) (actual time=1.535..1.535 rows=1 loops=1)
But if sort is performed by non-indexed fields, then current behaviour
will be inefficient and can be significantly improved by pushing limits
to remote hosts.
--
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company